This invention generally relates to casks for transporting radioactive materials, and is specifically concerned with an improved lightweight cask assembly having high strength titanium walls for transporting a maximum amount of radioactive material within a given weight limit.
Casks for transporting radioactive materials such as the waste products produced by nuclear power plant facilities are known in the prior art. The purpose of such casks is to ship radioactive wastes in as safe a manner as possible. Such casks may be used, for example, to ship high-level vitrified waste cannisters to a permanent waste isolation site or spent fuel rods to a reprocessing facility. At the present time, relatively few of such transportation casks have been manufactured and used since most of the spent fuel and other wastes generated by nuclear power plants are being stored at the reactor facilities themselves. However, the availability of such on-site storage space is steadily diminishing as an increasing amount of fuel assemblies and other wastes are loaded into the spent-fuel pools of these facilities. Additionally, the U.S. Department of Energy (D.O.E.) has been obligated, by way of the National Waste Policy Act of 1983, to move the spent fuel assemblies from the on-site storage facilities of all nuclear power plants to a federally operated nuclear waste disposal facility starting in 1998.
While the transportation casks of the prior art are generally capable of safely transporting wastes such as spent fuel to a final destination, the applicants have observed that there is considerable room for improvement, particularly with respect to vehicle-drawn, Type B casks. Specifically, the applicants have observed that the structural materials and design configuration used in these casks do not lend themselves to a maximum loading of radioactive wastes. The resulting less-than-maximum loading necessitates a larger number of trips by the shipper in order to complete the transportation of a given amount of radioactive waste, thus increasing both the time and the cost of transport. However, before the problems associated with maximizing the amount of waste carried by a particular cask may be fully appreciated, some understanding of the constraints imposed by U.S. government regulations is necessary.
U.S. Department of Transportation (DOT) and state highway regulations limit the gross weight of the waste carrying road vehicle to about 80,000 pounds for shipments without special permits. Since the typical tractor and trailer weighs approximately 30,000 pounds, the weight of a cask and its contents must not exceed approximately 50,000 pounds. These same regulations specify that the surface radiation of such cask be no greater than 200 millirems at any given point, and that the radiation emitted by the cask be no greater than ten millirems at a distance of two meters from the vehicle. Other DOT regulations require that the cask be capable of sustaining impact stresses of up to ten Gs in the longitudinal direction, five Gs in the lateral direction, and two Gs in the vertical direction without yielding the wastes. The end result of these regulations is that much of the 50,000 pounds must be expended in providing a wall structure that is dense enough to provide adequate shielding and strong enough to withstand the designated impact stresses. The resulting thickness of the wall necessary to provide the required radiation shielding and impact stresses leaves only a relatively small amount of space in the center of the cask which can actually be used to contain and transport radioactive waste. To maximize the amount of carrying volume, the most effective shielding materials known are frequently integrated into the walls of the cask structure. Such materials include lead, depleted uranium, and tungsten. However, as these materials are of a very high density, the radius of the cask walls cannot be made too large, or the gross weight limitation of 50,000 pounds of the combination of cask and waste material will be exceeded. Moreover, as U.S. government regulations require the cask design engineer to assume that such shielding materials have no structural strength and cannot be relied upon at all for compliance with the impact stress requirement, they must be integrated within structural walls which are capable of withstanding the designated stresses. At the present time, stainless steel is the most commonly used structural wall material. The end result of the foregoing constraints of structural strength, shielding effectiveness, and the high density of the most effective known shielding materials results in a very large portion of the 50,000 pounds weight allocation for a loaded cask going to the cask structure itself, rather than the weight of the waste being transported.
If the cost of transporting a particular amount of radioactive waste is to be minimized, then the weight of the cask structure relative to the weight of the waste being carried must be minimized. The applicants have further observed that this objective may be accomplished by the fulfillment of two criteria. First, the radial distance between the waste being carried and the shielding material integrated into the walls of the cask structure must be minimized. If this criteria is realized, an optimum shielding geometry results wherein a maximum amount of shielding is achieved with a minimum weight of shielding material. Second, the structural walls of the cask that overlie and support the shielding material should be fabricated from a material which affords maximum strength per unit weight of wall material. The applicants have further observed that, for many materials, these two criteria are incompatible with one another. Such incompatibility becomes evident when one considers that the interior surface of the shielding material must be lined with an inner structural wall in order to support the shielding material within the cask walls and to comply with the government impact stress regulations. If the distance between the waste and the shielding is to be minimized, then the material forming the inner wall must be as strong as possible per given thickness (or volume) of material. The thicker the material forming this wall is, the greater the distance between the waste and the shielding material, and the greater the radius (and hence weight) of the shielding material. Hence the use of a material such as a high-strength aluminum alloy would not necessarily result in any significant weight decrease of the cask as a whole. Even though such an alloy might be stronger than stainless steel on a pound-per-pound basis, and hence might reduce the weight of the outer structural wall, it would actually increase the weight needed for additional shielding material if the minimum thickness required for the inner wall was greater than the minimum thickness of the inner wall fabricated from stainless steel. The end result is that both of these weight reducing criteria are fulfilled only with a material that is substantially stronger than stainless steel both on a pound-per-pound and a volume-per-volume basis. Such a material would result in an outer structural wall of reduced weight, and would actually decrease the required amount of high density shielding material required to achieve the maximum surface radiation constraints.
Clearly, what is needed is a cask capable of containing a maximum amount of radioactive waste in a structure having a minimum amount of weight. Such a cask must also be capable of conducting and dissipating the heat of decay of the radioactive materials contained therein at least as well as cask wall structures made of stainless steel to avoid the creation of dangerous internal pressures. Finally, such a cask should be relatively simple and inexpensive to fabricate.